linux-kernel - [PATCH AUTOSEL 4.19 29/44] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue

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Message-Id: <20190311195700.138462-29-sashal@kernel.org>
Date:   Mon, 11 Mar 2019 15:56:45 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Jianchao Wang <jianchao.w.wang@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>,
        linux-block@...r.kernel.org
Subject: [PATCH AUTOSEL 4.19 29/44] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue

From: Jianchao Wang <jianchao.w.wang@...cle.com>

[ Upstream commit aef1897cd36dcf5e296f1d2bae7e0d268561b685 ]

When requeue, if RQF_DONTPREP, rq has contained some driver
specific data, so insert it to hctx dispatch list to avoid any
merge. Take scsi as example, here is the trace event log (no
io scheduler, because RQF_STARTED would prevent merging),

   kworker/0:1H-339   [000] ...1  2037.209289: block_rq_insert: 8,0 R 4096 () 32768 + 8 [kworker/0:1H]
scsi_inert_test-1987  [000] ....  2037.220465: block_bio_queue: 8,0 R 32776 + 8 [scsi_inert_test]
scsi_inert_test-1987  [000] ...2  2037.220466: block_bio_backmerge: 8,0 R 32776 + 8 [scsi_inert_test]
   kworker/0:1H-339   [000] ....  2047.220913: block_rq_issue: 8,0 R 8192 () 32768 + 16 [kworker/0:1H]
scsi_inert_test-1996  [000] ..s1  2047.221007: block_rq_complete: 8,0 R () 32768 + 8 [0]
scsi_inert_test-1996  [000] .Ns1  2047.221045: block_rq_requeue: 8,0 R () 32776 + 8 [0]
   kworker/0:1H-339   [000] ...1  2047.221054: block_rq_insert: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
   kworker/0:1H-339   [000] ...1  2047.221056: block_rq_issue: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
scsi_inert_test-1986  [000] ..s1  2047.221119: block_rq_complete: 8,0 R () 32776 + 8 [0]

(32768 + 8) was requeued by scsi_queue_insert and had RQF_DONTPREP.
Then it was merged with (32776 + 8) and issued. Due to RQF_DONTPREP,
the sdb only contained the part of (32768 + 8), then only that part
was completed. The lucky thing was that scsi_io_completion detected
it and requeued the remaining part. So we didn't get corrupted data.
However, the requeue of (32776 + 8) is not expected.

Suggested-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 block/blk-mq.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 23a53b67cf0d..7d53f2314d7c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -701,12 +701,20 @@ static void blk_mq_requeue_work(struct work_struct *work)
 	spin_unlock_irq(&q->requeue_lock);
 
 	list_for_each_entry_safe(rq, next, &rq_list, queuelist) {
-		if (!(rq->rq_flags & RQF_SOFTBARRIER))
+		if (!(rq->rq_flags & (RQF_SOFTBARRIER | RQF_DONTPREP)))
 			continue;
 
 		rq->rq_flags &= ~RQF_SOFTBARRIER;
 		list_del_init(&rq->queuelist);
-		blk_mq_sched_insert_request(rq, true, false, false);
+		/*
+		 * If RQF_DONTPREP, rq has contained some driver specific
+		 * data, so insert it to hctx dispatch list to avoid any
+		 * merge.
+		 */
+		if (rq->rq_flags & RQF_DONTPREP)
+			blk_mq_request_bypass_insert(rq, false);
+		else
+			blk_mq_sched_insert_request(rq, true, false, false);
 	}
 
 	while (!list_empty(&rq_list)) {
-- 
2.19.1